The present invention generally relates to a tape searching method in a digital audio tape recorder (hereinafter, referred to as DAT for short). More particularly, the invention relates to a high-speed searching for a desired tape portion by means of TOC (Table of Contents) and operation frequency of a reel motor.
Generally, a servo circuit used for the DAT system is employed to perform a tape searching function. Conventionally, the servo circuit is employed to perform a function of speed-variable play such as normal speed (.times.1), two-speed (.times.2), three-speed (.times.3), five-speed (.times.5), nine-speed (.times.9) and sixteen-speed (.times.16) play and high speed searching of one-hundred-speed (.times.100) and two-hundred-speed (.times.200). However, a demand for tape searching faster than the two-hundred-speed play arises gradually, as the technique in the DAT system is advanced. Particularly, in order to read out data stored in the data-storage tape with higher speed, there arise a need to search the tape with a speed higher than the two-hundred-speed.
But, in the case where a tape searching faster than two-hundred-speed is enforced according to the method heretofore in use, it is very difficult to detect a start ID (identification) of each program written in the tape, while keeping the relative velocity of the drum constant. As wherein, the relative velocity is found by following a procedure causing undesirable drawbacks.
Hereinafter, the problems of the prior DAT system, which occurred during fast tape searching will be explained assuming that the DAT system has a drum diameter of 30 mm, a rotational speed of 2000 rpm (in normal play), a tape running speed of 8.15 mm/sec and a track pattern (still) slope of 6.degree.22'.
When a fast forward (hereinafter called FF for short) search is made at two-hundred-speed (.times.200) play, the relative velocity VR of the drum is given as EQU VR=X.multidot.Vd.multidot.cos .theta.-200Vt (1)
Wherein, X represents a magnification factor of the present drum speed against the normal drum speed, Vd normal speed of the drum and Vt normal tape running time.
In order to obtain the magnification factor X, if the left term of Equation (1) is substituted by a normal drum speed, then EQU Vd.multidot.cos .theta.-Vt=X.multidot.Vd.multidot.cos .theta.-200Vt(2)
From the above Equation (2), the magnification factor X can be obtained as ##EQU1##
In Equation (2), it is noted that the number of drum rotations is 1.52 times that of the normal speed, which is 3040 rpm, if the tape is searched at two-hundred-speed FF search mode.
Conversely, when a rewind (hereinafter, called RWD for short) search is made at two-hundred-speed (.times.200), the tape running speed Vt becomes negative number. Therefore, the number of drum rotations for keeping the relative velocity VR constant must be 960 rpm upon the basis of the following Equations (4) and (5). ##EQU2##
For a three-hundred-speed (.times.300) FF search, the magnification factor X must be X.apprxeq.1.78 in the same way, the number of drum rotation must be about 3,560 rpm with X.apprxeq.1.78 and, when a search is made at three-hundred-speed (.times.300) RWD, the number of drum rotation must be about 420 rpm with X.apprxeq.0.2146.
Considering such a point, the performance of a drum motor is limited to a specific speed, as well illustrated in FIG. 2. That is to say, if the tape searching is made at or close to the four-hundred-speed (.times.400) RWD mode, the drum must be rotated backwardly theoretically. In practice, however, it is impossible for the drum to do so. It is also difficult to make a high speed search by correcting only the relative velocity of drum in such a case, and problems arise as to how to contrive another method of searching the tape with high speed.